MEMS and other devices fabricated on wafer substrates are typically bonded to protective covers or caps using techniques such as fusion bonding, anodic bonding or adhesive bonding. In general, the bond is made with the parts to be bonded being held in some type of clamping fixture, sometimes within a vacuum chamber. In situ inspection of the bond formed between the cap and the substrate is problematic, especially in cases where the cap and substrate are formed of materials such as silicon, that are opaque in the visible spectrum. Inspection has to occur after the capped device is removed from the vacuum chamber and/or fixture, and if the bond quality is then found to be poor, the bonding process has to be repeated or the device has to be rejected.
It is therefore desirable to provide a method for bonding a cap to a substrate-based device which allows in situ optical inspection of the bond to be carried out during or immediately after the bonding process. It is also desirable that the method should allow, if an initial bond is observed to be unsatisfactory, for another nearby bond between portions of that same device and cap to be quickly and conveniently formed and inspected in situ.
The established bonding techniques mentioned above have various drawbacks in addition to those related to inspection. Fusion bonding imposes strict requirements on surface flatness and cleanliness over significantly large areas, and involves the application of considerable heat and pressure, often provided by some type of clamp or weight directly contacting the cap and/or the substrate. Such contact may cause damage or even fracture the device cap. Anodic bonding is restricted to a relatively narrow material set, and generally requires high temperatures that may be detrimental to the underlying circuitry, and may increase the probability of thermal expansion mismatch-related bond failure. Achieving bubble-free hermetic bonds with adhesives is very difficult if not impossible. Bubbles increase the probability of bond failure and adhesives do not bond well to hard smooth materials. It is therefore desirable to provide a method that creates a bond over a relatively small bonding surface area, as this reduces the likelihood of the bond being affected by surface contamination and roughness. It is also desirable that the method should avoid imposing direct contact between any clamping or positioning fixture and the top surface or clear aperture of the cap, maintain as much as possible of the cap and substrate at room temperature throughout the bonding process, and avoid the formation of bubbles at the bond site.